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Int J Cancer. 2006 Sep 15;119(6):1468-74.

Family history of breast cancer and short-term effects of childbirths on breast cancer risk.

Author information

1
Department of Mathematics, University of Bergen, Bergen, Norway. msega@uib.no

Abstract

The long-term protective effect of a pregnancy on breast cancer risk is preceded by a short-term adverse effect, possibly reflecting a promoting effect of pregnancy hormones. In the present study, we explore whether a family history of breast cancer modifies time-related effects of pregnancies, with special emphasis on the transient increase in risk of breast cancer shortly after birth. Our study cohort comprises 1,067,289 Norwegian women aged 20-74 years. The mean follow-up time was 18 years. Incidence rate ratios were estimated by Poisson regression analyses of person-years at risk. Of the 7,377 women diagnosed with breast cancer during follow-up, a total of 828 (11%) had a mother or a sister with breast cancer diagnosis. Women with a family history of breast cancer had a 2-3-fold higher risk of breast cancer than did women without any affected family member, highest for those with a relative diagnosed before they were 50 years. Similar to women without a familial excess risk, increasing parity was associated with an overall protective effect among women with a familial predisposition, regardless of age at diagnosis of the relative. Whereas women with no familial excess risk experienced a transient increase in risk mainly after late age births, women with a family history of breast cancer experienced an adverse effect of pregnancies also at younger ages. The present results give further support to the hypothesis that the adverse effect of a term birth can be explained by a promoting effect of pregnancy hormones.

PMID:
16642468
DOI:
10.1002/ijc.22003
[Indexed for MEDLINE]
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